Back to EveryPatent.com
| United States Patent |
5,252,187
|
|
Ohtsu
, et al.
|
October 12, 1993
|
Method of recovering solvent from mother liquor containing non-volatile
matters by heat pump system
Abstract
A method of recovering a solvent from a mother liquor in a system in which
the mother liquor also contains non-volatile matter, volatile solute and
volatile solvent, and the solute concentration is low, in which a heat
pump distilling column (first column) is so operated as to work only as a
concentrating section of a distilling column by feeding a mother liquor to
the column bottom of the heat pump distilling column, contact of
non-volatile matter with the concentrating section is prevented and the
column bottom liquor is sent to an evaporator so as to separate the
non-volatile matter by the evaporator, thereafter, a vapor from the
evaporator is fed to a distilling column (second column), and distillates
of first and second columns and bottoms of the second column are withdrawn
as the products. The procedures described above are carried out under
specific operation conditions.
| Inventors:
|
Ohtsu; Jun (Tokyo, JP);
Abe; Kanji (Chiba, JP);
Ida; Toshifumi (Chiba, JP)
|
| Assignee:
|
Toyo Engineering Corporation (Tokyo, JP)
|
| Appl. No.:
|
918453 |
| Filed:
|
July 22, 1992 |
Foreign Application Priority Data
| Jul 25, 1991[JP] | 3-186207 |
| Sep 11, 1991[JP] | 3-231714 |
| Current U.S. Class: |
203/26; 159/24.2; 159/47.1; 159/DIG.10; 203/27; 203/78; 203/84; 203/DIG.4; 203/DIG.8; 203/DIG.9 |
| Intern'l Class: |
B01D 001/28; B01D 003/00 |
| Field of Search: |
203/26,14,27,DIG. 8,DIG. 9,89,78,84,72,78,DIG. 4
159/49,5,13.1,24.2,DIG. 32,DIG. 10,47.1
260/DIG. 22
|
References Cited
U.S. Patent Documents
| 4177111 | Dec., 1979 | Pieper et al. | 203/26.
|
| 4539076 | Sep., 1985 | Swain | 203/26.
|
| 4559108 | Dec., 1985 | Ahlberg | 203/26.
|
| 4566947 | Jan., 1986 | Tsuruta | 203/DIG.
|
| 4615769 | Oct., 1986 | Horigome et al. | 203/26.
|
| 5124004 | Jun., 1992 | Grethlein et al. | 203/DIG.
|
| Foreign Patent Documents |
| 0209602 | Dec., 1982 | JP.
| |
Primary Examiner: Manoharan; Virginia
Attorney, Agent or Firm: Flynn, Thiel, Boutell & Tanis
Claims
What we claim is:
1. A method of recovering a solute and a solvent from a mixture which
contains a volatile solute, a volatile solvent and non-volatile matter,
and in which said solute and said non-volatile matter are present in low
concentrations, comprising the steps of: feeding a mother liquor to the
column bottom of a first distillation column so that said first
distillation column operates only as a concentrating section of a
distillation column and said non-volatile matters are prevented from
coming into contact with said concentrating section, flowing the column
top vapor from said first distillation column through a compressor and
thence through a reboiler, heating a portion of the column bottom liquid
from said first distillation column in said reboiler and then returning
said portion to said column bottom of said first distillation column,
feeding the remainder of the column bottom liquid of said first
distillation column to an evaporator and therein evaporating said solute
and said solvent while maintaining said non-volatile matter in a non-vapor
condition, separating said non-volatile matter in said evaporator, feeding
the vapor from said evaporator to a second distillation column, and
withdrawing distillates of said first and second columns and the bottoms
of said second distillation column as products, wherein the steps
described above are carried out under operation conditions satisfying the
following relations:
1. the compression ratio of said for the column top vapor of said first
distillation column: Pd/Ps.ltoreq.10, wherein Pd=compressor discharge
pressure, Ps=compressor intake pressure, and
2. the temperature difference between the saturation condensation
temperature (Tdb) of the vapor at the compressor discharge and the boiling
point (Tbb) of the bottom liquid of said first distillation column:
Tdb-Tbb.ltoreq.30.degree. C.
2. The method of claim 1 where Pd/Ps is from 1.1 to 3.0 and Tdb-Tbb is from
3.degree. to 20.degree. C.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of recovering a solvent by a heat
pump system. More particularly, it relates to a method of recovering a
solvent which method will be useful in industrial fields requiring the
recovery and re-utilization of volatile solutes and volatile solvents
present in a mother liquor, in a system in which non-volatile solids or
liquids also are dissolved in the mother liquor.
2. Description of the Related Prior Art
In the fields of industrial chemistry, petrochemistry, pharmaceuticals,
foodstuffs, etc., volatile solutes and volatile solvents in a mother
liquor have been recovered and re-utilized, in many cases from a system in
which non-volatile solids or liquids also are dissolved in the mother
liquor. Various methods of recovering the solvents have been proposed in
the past as described below.
(1) Vapor re-compression system distillation method:
This system is referred to as a "heat pump distillation system" and has
already been put into practical use in the fields of petrochemistry and
foodstuffs as an energy-saving, economical distillation method for
reducing the heat input required for effecting distillation (e.g.,
Japanese Patent Laid-Open No. 209602/1982).
(2) Evaporation technique:
This technique has been put into practical use as a technique for
vaporizing and separating a solvent when non-volatile solids or liquids
are dissolved in a volatile liquid. A large number of evaporators have
been proposed.
(3) Multiple effect evaporation technique:
When the concentration of the solutes is low and large quantities of heat
input are necessary in order to vaporize and separate the solvent, this
technique sequentially connects a plurality of evaporators and
sequentially causes evaporation in the evaporators by the use of the vapor
generated by the evaporators of the preceding stages. This technique has
already gained a wide application as a technique for reducing the heat
input required in industrial fields.
(4) Continuous rectification technique not having recovery section:
This technique is referred to as a "still head" which has a rectifier
having a fractionating column at a still section, and has conventionally
been used as a batch-wise or continuous rectification technique.
Among them, the vapor re-compression system distillation method and the
multiple effect evaporation technique have been used primarily, in view of
the energy savings they provide.
However, when applied to the solvent recovery method which is the object of
the present invention, the conventional vapor re-compression system
distillation method and the multiple effect evaporation technique are not
free from the following problems. According to the conventional vapor
re-compression system (heat pump system) distillation method, the
concentrating (rectifying) section and the recovery (stripping) section of
the distilling column are necessary in order to recover the column top
fraction and the column bottom fraction, respectively. Hence, the mother
liquor (feed material) must be fed to an intermediate location between the
concentrating section and the recovery section. When the mother liquor
contains a non-volatile solid and liquid, therefore, the non-volatile
solid and liquid must flow down through the packing materials and trays of
the recovery section, and this results in contamination and clogging of
the column and eventual failure of the operation.
The multiple effect evaporation technique can process the mother liquor
containing the non-volatile solids and liquids and can reduce the
necessary heat input, but it cannot withdraw products having a specified,
high purity. The combination technique with the "still head" can be
employed to withdraw distillate products having a high purity, but a large
number of evaporators equipped with the "still head" are necessary in
order to withdraw distillate and bottom products having high purities,
while reducing the heat input. In other words, in order to reduce the
necessary heat input, the multiple effect evaporation technique needs a
large number of effect columns and presents the problem that the cost of
the installation is increased.
SUMMARY OF THE INVENTION
The present invention provides a solvent recovery method by a heat pump
system, which method can withdraw distillate products and bottom products
having high purities while reducing the necessary heat input and can use a
simple apparatus.
The present invention relates to a method of recovering solutes and a
solvent by a heat pump system from a working system which contains a
volatile solute, a solvent and a non-volatile solute and, moreover, in
which the solutes are present in low concentrations. The method comprises
the steps of feeding a mother liquor to the column bottom of a heat pump
distilling column (first column) so as to permit the heat pump distilling
column to operate only as a concentrating section of a distilling column,
thereby preventing the non-volatile matters from coming into contact with
the concentrating section, sending the column bottom liquid of the first
column to an evaporator, separating the non-volatile matters by means of
the evaporator, feeding the vapor from the evaporator to a conventional
distilling column (second column), and withdrawing distillates from the
first and second columns and bottoms from the second column as the
products, the method being characterized by the features that the steps
described above are carried out under operation conditions satisfying the
following relations:
1. The compression ratio (absolute pressure ratio) of the compressor for
the column top vapor of first column: Pd/Ps.ltoreq.10 (wherein
Pd=compressor discharge pressure; and Ps=compressor intake pressure), and
2. The temperature difference (at the reboiler) between the saturation
condensation temperature (Tdb) of the vapor at the compressor discharge
and the boiling point (Tbb) of the liquid at the lower par of
concentrating section as the bottom liquid of the heat pump distilling
column: Tdb-Tbb.ltoreq.30.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a typical example of a recovery flow according
to the present invention.
FIG. 2 is a diagram showing a 5-column type flow as a typical example of a
recovery flow according to the conventional multiple effect column.
FIG. 3 is a diagram showing an example where the conventional vapor
re-compression system distillation method is applied to the solvent
recovery method, for comparison with the present invention.
In FIGS. 1, 2 and 3, numeral 1 identifies a supply of a mother liquor,
numerals 3 and 22 identify column tops, numeral 5 identifies a vapor
compressor, numerals 9, 11 and 18 identify conduits, numerals 12 and 28
identify reboilers, and numeral 15 identifies an evaporator.
DETAILED DESCRIPTION OF THE INVENTION
The reasons for the limitations according to the above-described operation
conditions are as follows:
(i) The smaller the compression ratio (Pd/Ps) of the compressor 5, the
smaller is the amount of power (work energy) that must be supplied to
operate the compressor, that is, the input (work) energy is reduced. If
this ratio is too small, however, the temperature difference (Tdb-Tbb) in
the reboiler 12 becomes excessively small, so that the heat transfer area
of the reboiler must be increased and the cost of the reboiler will be
increased as well.
(ii) To achieve the desired purity of the distillates, a minimum reflux
ratio and a minimum number of theoretical plates exist, and the reflux
ratio and the number of theoretical plates are synergistic, mutually
interrelated functions of the desired purity of the distillates. In other
words, there is the relation such that if one of (1) the minimum reflux
ratio or (2) the minimum number of theoretical plates, is increased to
achieve a predetermined purity of the distillates, the other is decreased.
In connection with the work energy supplied to operate the compressor, on
the other hand, both of (1) increase in the number of theoretical plates
and (2) an increase in the reflux ratio, lead to an increase in the work
energy. This means that there is a minimum work energy value in order to
achieve a predetermined distillate purity.
From the aspect of economy of conventional distilling column equipment,
there is a method which sets a reflux ratio at from 1.1 to 1.5 times the
minimum reflux ratio. However, an increase in the reflux ratio leads to an
increase in the quantity of the fluid that passes through the compressor 5
and, thus, results in an increase in the power required to operate the
compressor. Accordingly, in order to limit the power required to operate
the compressor, it is necessary to minimize the reflux ratio as much as
possible. From the relation of the synergistic functions described above,
however, the reduction of the reflux ratio means an increase in the number
of theoretical plates. An increase in the number of theoretical plates
leads to an increase in the number of stages or the height of the packing
inside the column and results in an increased pressure drop inside the
column. Accordingly, the pressure difference between the column top and
the column bottom becomes greater and results in an increase of the
compression ratio (Pd/Ps) of the compressor, described in paragraph (i).
Therefore, to limit the power of the compressor while economically
restricting the reboiler heat transfer area described in paragraph (i),
the discharge pressure of the compressor is set to a pressure at which the
discharged vapor has a saturation condensation temperature (Tdb) which is
from 3.degree. to 20.degree. C. higher than the boiling point (Tbb) of the
column bottom liquid.
For the reasons described in paragraphs (i) and (ii), the compression ratio
of the compressor 5 and the temperature difference between the saturation
condensation temperature (Tdb) of the vapor at the compressor discharge
and the boiling point (Tbb) of the liquid at the lower part of the
condensation section are limited as described above. Particularly, the
compression ratio is preferably set to be within the range of 1.1 to 3.0
and the operation condition is preferably selected so that Tdb-Tbb is from
3.degree. to 20.degree. C.
However, if there is a maximum limit to the heating temperature, because of
the temperature characteristics, such as the decomposition temperature of
the volatile solvent, the volatile solute or the non-volatile solids or
liquids, the operation pressure of the column must be selected so that the
temperature is below this maximum temperature.
DESCRIPTION OF PREFERRED EMBODIMENT
Hereinafter, a solvent recovery method by a heat pump system according to
the present invention will be explained with reference to the accompanying
drawings.
FIG. 2 shows a 5-column type flow as a typical example of the recovery flow
by a multiple effect evaporation technique of the prior art. A
fractionating section is disposed at the upper part of each effect column.
The operation pressure of each effect column is reduced gradually toward
the downstream side. When the distillate is constant, the evaporation
quantity of each effect column is substantially equal. When the number of
columns of the multiple effect column is n, the necessary quantity of
externally supplied heat medium, such as steam, is 1/n of that for a
single column. The bottoms are a mixture containing non-volatile matters
and volatile solvent. Therefore, to separate and purify further the
volatile solvent, an apparatus for the separation and purification must be
provided separately.
FIG. 1 shows a typical example of the recovery flow according to the
present invention.
Mother liquor 1 is supplied to the column bottom of a column A
(corresponding to the first column). This mother liquor 1 is preferably
pre-heated to a temperature near its boiling point. It can be pre-heated
through heat-exchange with the distillates 26, 31 or with the bottoms 30.
Prior art techniques can be employed for this purpose. The column A has
only a portion 2 corresponding to a concentrating (rectifying) section of
a distilling column. This section 2 may be either a packed column or a
tray column, and a column having a small pressure drop inside it is
greatly preferred. The vapor flows through a column top line 4 of the
column A and enters a vapor compressor 5, where the temperature and
pressure of the vapor are raised. If the heat energy of the vapor
discharged from the compressor 5 is excessive, depending on the condition
of heating, a part of the vapor can be flowed through a line 8 and a
regulating cooler 13. All of the vapor, except that flowed through line 8,
is flowed through a discharge line 7 and a reboiler 12. This vapor
provides the necessary heat energy to achieve the separation in the column
A. In other words, the vapor from the top of column A is thermally coupled
to the bottoms reboiler 12. The compressor 5 uses shaft work to elevate
the energy of the vapor to a temperature high enough for reboiling
purposes. The condensates from the reboiler 12 and the regulating cooler
13 are combined and a portion of the combined condensates is returned to
the column A through a reflux line 10. The remainder of the combined
condensates is withdrawn as the distillate 31. A portion of the bottom
liquid from column A is flowed though line 11 into the reboiler 12 wherein
it is vaporized and returned to the column. The remainder of the bottom
liquid of the column A passes through a line 14 and is sent to an
evaporator 15. The non-volatile matter initially present in the mother
liquor 1 is withdrawn, as a liquid, from the evaporator through a line 17.
The mixed vapor of the volatile solute and the volatile solvent evaporated
in the evaporator 15 is supplied through a line 16 to a feed stage section
19 of a column B (corresponding to the second column).
Examples of evaporators that can be used as the evaporator 15 include a
thin film evaporator, a falling film evaporator or a kettle type
heat-exchanger. The most suitable evaporator is selected in accordance
with the properties of the non-volatile matter. When the non-volatile
matter must be withdrawn in a high concentration and, moreover, when it is
viscous, for example, it is preferred to use a thin film evaporator. The
evaporation conditions are determined in accordance with the concentration
of the non-volatile matter and the maximum temperature to which it can be
heated safely.
The condition of pressurization of the vapor by the compressor 5 is
determined as already described in the paragraph (i). In this case, the
compressor can be selected from various known types, such as a turbo
compressor, a screw compressor, a reciprocation compressor, and so forth.
The discharge temperature of the vapor from the compressor 5 is determined
by the type of compressor used, the compression ratio and the intake
condition, but the vapor is generally in a superheated state and has a
higher heat energy in many cases than the heat energy that is required for
heat transfer to the bottoms in the reboiler 12. Accordingly, the
regulating cooler 13 is provided, generally on the discharge side of the
compressor. A temperature controller 40 can be provided in the superheated
vapor line 6 or 7 in order to take into account the heat transfer area of
the reboiler 12 and the maximum temperature to which the bottoms are
heated in the reboiler 12.
The column B is a conventional distilling column having a concentrating
(rectifying) section 20 and a recovery (stripping) section 21. The column
top vapor enters a condenser 25 via a line 23. After the vapor is
condensed, a part of it is returned to the column B via a reflux line 24
while the remainder is withdrawn from the column B as the distillate 26.
The distillates 26 and 31 can be either mixed or withdrawn individually,
depending on the product specification for the distillates. A portion of
the column bottom liquid from column B is flowed through line 27 to a
reboiler 28. The reboiler 28 of the column B provides the necessary
thermal energy to the column bottom liquid to effect the separation in the
column B. The thermal energy is supplied to reboiler 28 from an externally
supplied heating medium. A line 29 can be provided for returning another
portion of the column bottom liquid to the evaporator 15 because a trace
amount of the non-volatile matter from the evaporator 15 may be mixed with
the vapor fed into the column B, due to entrainment. The remainder of the
bottoms are withdrawn as vapor from a line 50 or as a liquid from a line
51.
As an example of the application of the conventional vapor recompression
type distillation method to the solvent recovery method, which is the
object of the present invention, as shown in FIG. 3, it may be possible to
place an evaporator in the mother liquor feed conduit for the first
column, to supply the heat energy for this evaporator by the heat pump
system and to separate beforehand the non-volatile matters. However, this
system is not preferable for the following reasons. First, evaporation and
separation of large quantities of components of the mother liquor are
necessary and, hence, the size of the evaporator installation becomes
large. Next, both the concentrating section and the recovery section are
provided in the distilling column equipped with the heat pump, so that the
pressure drop increases and the boiling point of the bottoms rises
unavoidably. If any maximum temperature limit exists for the solute,
flexibility of operation is lost. Moreover, since the pressure on the
discharge side of the compressor increases, the power must be increased.
Furthermore, the distillation effect of the recovery section of the
distilling column equipped with the heat pump is limited because a
considerable portion of the heat source capacity of the discharged vapor
of the compressor must be distributed to the evaporator.
As can be understood clearly from the explanation given above, the solvent
recovery method by the heat pump system according to the present invention
provides the following remarkable effects:
(1) The portion ranging from the column top of the heat pump distilling
column A to the reboiler 12 is used as the concentrating section of the
distilling column, and the column bottom liquid of the heat pump
distilling column is supplied to the evaporator 15. Since this system is
employed, contamination and clogging of the packing materials inside the
column and the tray can be prevented. Accordingly, rectification and
separation of the solution containing the non-volatile liquid or the solid
can be carried out efficiently.
(2) The portion ranging from the column top of the heat pump distilling
column A to the reboiler 12 is used as the concentrating section of the
distilling column, and the vapor from the evaporator 15 can be supplied to
a conventional distilling column (including columns that operate under
reduced or elevated pressures). Therefore, rectification of the
distillates and the bottoms can be achieved easily. The invention can be
applied to separating operations having a strict purity specification.
(3) The number of columns can be smaller than in the conventional multiple
effect column, the structure of the columns can be much more simplified
than in the multiple effect distillation, and the cost of installation can
be reduced, too.
(4) The quantity of the necessary externally supplied heat energy, for
example, steam, can be reduced drastically and eventually the operation
cost can be reduced, too.
EXAMPLES
Hereinafter, an example and a comparative example will be given so as to
explain the present invention in more detail.
EXAMPLE 1 AND COMPARATIVE EXAMPLE 1
A comparative test was carried out between (1) a conventional multiple
effect column system, as shown in FIG. 2, and (2) the system of the
present invention shown in FIG. 1, for a system in which (1) DMAC
(N,N-dimethylacetoamide), as the solute, and (2) polymer, as the
non-volatile matter, were dissolved as a mixture in a large quantity of
water, as a solvent.
______________________________________
Composition of mother liquor:
______________________________________
H.sub.2 O 94.9 wt %
DMAC 5.0 wt %
polymer 0.1 wt %
______________________________________
The recovered materials were water, as the distillate, and DMAC, as the
bottoms, and each had a purity of more than 99.9 wt %. The polymer was
withdrawn as a mixture with DMAC in both of these systems. The operation
conditions of each system are tabulated in Table 1.
Comparisons of energy consumption and energy cost were carried out under
the operation conditions shown in Table 1. The results are tabulated in
Table 2. In order to standardize the energy level of a required quantity,
the necessary heat in the distillation operation necessary for bringing
the concentration of the DMAC bottoms of the multiple effect column system
into conformity with the level of the present system was added in Table 2.
The calculation was conducted, based on a feed quantity of 20,000 kg
mother liquor per hour.
TABLE 1
__________________________________________________________________________
Conventional multiple effect
This system
column system
1st 2nd 1st 2nd 3rd 4th 5th
column
column
column
column
column
column
column
__________________________________________________________________________
Column top pressure
5.5 1.2 4.5 2.7 1.2 1.0 0.1
(kgf/cm.sup.2 A)
column top 155 104 147 129 104 99 45
temperature (.degree.C.)
reflux ratio
0.7 1.0 1.1 1.0 0.9 0.9 1.0
distillate 99.9
99.9
99.9
99.9
99.9
99.9
99.9
concentration
H.sub.2 O (wt %)
compressor discharge
7.7
pressure (kgf/cm.sup.2 A)
bottom concentration
99.9 67.0
DMAC (wt %)
__________________________________________________________________________
TABLE 2
______________________________________
Conventional
multiple effect
This system
column system
______________________________________
required energy
quantity
steam (T/H) 2.4 10.4
electricity 1,100 220
(KWH/H)
cooling water 220 490
(m.sup.3 /H)
operation cost
30,300 38,050
(yen/hr)
______________________________________
energy unit price:
steam: 3,000 yen/ton
electricity: 20 yen/KWH
cooling water: 5 yen/m.sup.3
Top